Tom yum soup

Name: Tom yum soup
Cook time: 30 min
Total time: 30 min
Cuisine: Asian
Ingredients: 750ml chicken stock; 1 lemongrass, bashed; a thumb-sized piece ginger, thinly sliced; 1 bird's-eye chilli, halved; 2 cloves garlic, bashed; 2 torn lime leaves, 1 thinly sliced to serve (see notes below); 2 plum tomatoes, roughly chopped; 125g button mushrooms, quartered; 150g raw peeled prawns; 1 tbsp soft light brown sugar; 1 lime, juiced; 1 tbsp fish sauce